Trailing whitespace

```haskell
> pPrintString "Union (Cat (Char 'x') Fail) (Cat (Char 'x') Line)"
Union 
    ( Cat ( Char 'x' ) Fail ) 
    ( Cat ( Char 'x' ) Line )
```

There's a trailing space character both on the first and second line. It would be nice if `pretty-simple` wouldn't do this, although it hasn't been a big problem for me so far.